This invention relates generally to heat transfer systems. Specifically, this invention relates to microscale Stirling cycle devices, which may be used to heat, cool, or maintain a steady temperature in associated items such as electronics, sensors, microelectromechanical system (MEMS) devices, or spacecraft components.
The development of electronics and microelectromechanical systems has been accompanied by the need for systems of similar small size to control the temperature of these items. Some of the systems that have been used for temperature control of devices of any size have included natural convection enhancements, conduction enhancements, radiation enhancements, forced air systems, liquid cooling loops, heat pipes, thermoelectric devices, standard thermodynamic cycle devices, resistance heaters, combustion heaters, and various combinations thereof. Devices of these types have historically been large, when compared to the sizes of microelectronic or MEMS devices. It has been difficult to miniaturize many of these traditional heating and cooling devices. This is because the material and means of manufacture of the traditional components of these devices, such as the pistons, linkages, and pressure vessels of a traditional Stirling cooler, for example, are generally not suited for microscale production.
In addition to the difficulties of miniaturization, most heat transfer systems act either to cool or to heat, but not both. This limitation applies both to microscale and traditional scale thermal control devices. Under circumstances when it was necessary to alternately heat or cool a device it has commonly been necessary, or most efficient, to perform the heating and cooling tasks with separate or multiple devices. This also applies to circumstances when it is necessary to precisely maintain the temperature of a device.
One system that has been employed for cooling utilizes the Stirling cycle. One embodiment of a Stirling cycle for refrigeration uses two pistons to create a temperature difference between one end of the system and the other. Each piston is made to oscillate sinusoidally. The operation of the pistons is out of phase with each other. A typical phase shift in a functional device is generally between 90 degrees and 120 degrees. Initially, the working gas is compressed by two pistons moving in opposite directions toward each other or by one piston moving toward a second fixed piston. Ideally, compression occurs isothermally with the working gas transferring just enough heat to the surrounding cylinder walls so that the temperature remains constant as the pressure increases and the volume decreases. The working gas is then moved through a regenerator region by the parallel motion of the two pistons. As the working gas moves through a regenerator region, it transfers more heat to the cooler regenerator.
Next, the cooled gas is expanded by the pistons moving in opposite directions or by one piston moving away from a second fixed piston. Ideally, this expansion occurs isothermally, with the working gas accepting just enough heat from the cylinder walls so that the temperature remains constant as the pressure decreases and the volume increases. The working gas is then moved back through the regenerator by the parallel motion of the pistons. As the gas moves through the regenerator, it accepts additional heat from the warmer regenerator.
As the cycle continues, the temperature difference between the warm side of the system and the cool side of the system increases. The temperature of each side eventually approaches a steady state temperature.
Stirling cycle heat transfer systems have generally been too large for use with MEMS and other small electronic devices. Previous attempts to miniaturize Stirling cycle coolers were limited by increased thermal conductivity though the regenerator region as the frequency of oscillations increased, the decreased exchange of thermal energy between the walls of the regenerator region, and the increasing viscosity of the working fluid. Bowman, et al., U.S. Pat. Nos. 5,457,956, 5,749,226, and 5,941,079, have attempted to apply Stirling cycle principles to a micro-scale cryocooler. The Bowman Stirling cryocooler utilizes small regenerator passages and high frequency, low amplitude oscillations to overcome some of the problems associated with miniaturizing a Stirling cooler.
While Bowman represents an advance, it still does not solve many of the problems associated with the temperature control of MEMS. Some MEMS require both heating and cooling. The Bowman device functions as a cooler. In addition, Bowman teaches an unmoderated means for cooling. There is often a need to precisely control temperature in a manner that cannot be achieved by Bowman. For example, a microscale device including biological material may need to be held within a very narrow temperature range. As a second example, certain temperature sensitive sensors of microscale or larger size may need to be held within a very narrow temperature range, in order to produce accurate readings. Also, most applications requiring cooling or temperature control operate well above the cryogenic temperature range of the Bowman device. Finally, the. Bowman devices must be custom designed for a particular application.
Thus, there still exists a need for an active thermal control device that can be produced in a microscale size. In addition there still exists a need for a single thermal control device that selectively heats, cools, and can precisely control temperature. Further, there still exists a need for a microscale cooler that operates at temperatures above the cryogenic range. Finally, there still exists a need for a modular thermal control device that can be used in conjunction with other modular thermal control devices to closely fit the unique surfaces of each of a plurality of items that require temperature control.
It is an object of an exemplary form of the present invention to provide a modular thermodynamic device that is capable of both heating and cooling an associated device.
It is a further object of an exemplary form of the present invention to provide a modular thermodynamic device that is capable of achieving and maintaining a designated steady state temperature in the associated device.
It is a further object of an exemplary form of the present invention to provide a modular thermodynamic device that is capable of being used in series with other modular thermodynamic devices to provide wider range of temperatures to which the associated device can be heated or cooled.
It is a further object of an exemplary form of the present invention to provide a modular thermodynamic device that is capable of being used in parallel with other modular thermodynamic devices to provide a capacity to heat or cool a relatively larger surface area than the surface of a single modular thermodynamic device.
It is a further object of an exemplary form of the present invention to provide a modular thermodynamic device that is capable of being used both in parallel and in series with other modular thermodynamic devices to heat or cool a relatively larger surface area over a wider range of temperatures, as compared to the range of a single modular thermodynamic device.
It is a further object of an exemplary form of the present invention to provide a modular thermodynamic device that is capable of heating or cooling a curvilinear surface.
It is a further object of an exemplary form of the present invention to provide a modular thermodynamic device which can be used in series with other modular thermodynamic, the series combination being capable of heating or cooling a curvilinear surface to a relatively wider range of temperatures than that is generally achieved by the use of a single modular thermodynamic device.
Further objects of an exemplary form of the present invention will be made apparent in the following Best Modes for Carrying Out Invention and the appended claims.
The foregoing objects are accomplished in an exemplary embodiment of the invention by a microscalable temperature control module that utilizes the Stirling cycle. The microscalable temperature control module is operative to create a temperature difference between two thermal energy transfer layers. One of the thermal energy transfer layers is in contact with an associated device that is to be heated, cooled, or maintained at a selected temperature. The microscalable temperature control module either transfers heat to, or receives heat from, the surface of the associated device. Through this heat transfer, the temperature of the associated device approaches the same temperature as the thermal energy transfer layer with which it is in contact.
When used in isolation, an exemplary embodiment of a microscalable temperature control module has a temperature sensor on one thermal energy transfer surface to monitor the surface temperature of the device. The temperature sensor is electronically linked to a controller that is operative to control the phase of the oscillations of the microscalable temperature control module. By shifting the phase of the oscillations, such a microscalable temperature control module can be electronically switched from heating to cooling, or vice versa. By the use of feedback from the temperature sensor to switch such a microscalable temperature control module from heating or cooling as needed, very precise temperature control can be maintained. Because the surface of such a microscalable temperature control module is in contact with the surface of the associated device, the associated device will approach the same temperature as the surface with which it is in contact.
In alternative embodiments, multiple copies of the microscalable temperature control module can be configured so as to increase the range of temperatures at which the steady state temperature can be maintained. Such embodiments may also be suitable to increase the surface area over which temperature can be controlled. Alternative embodiments may also enable temperature control of a curvilinear surface or other regular or irregular surface contour. When used in multiples, the microscalable temperature control modules may be electronically controlled by the same temperature sensor.